1. Introduction
Solar Spectral Irradiance (SSI) in EUV wavelengths originates in the solar corona and the transition region. It provides information about the physical, chemical, thermal, and dynamical properties of the solar atmosphere. Solar emission in EUV wavelengths is the primary source of energy input to planetary atmospheres and plays an indispensable role in the atmospheric structure and composition, e.g., it has an essential role in the Earth’s atmosphere, driving the dynamics in the Ionosphere-Thermosphere-Mesosphere (ITM) system (Rodríguez Gómez Reference Rodríguez Gómez2025a and references therein).
SSI models are important mainly due to observational gaps. In general, these models arise from theoretical and empirical descriptions. Since the photospheric magnetic flux drives the solar irradiance variability, it is possible to use the solar magnetic field to model the Solar Spectral Irradiance (SSI), e.g., the COronal DEnsity and Temperature (CODET) model, a physics-based model (Rodríguez Gómez Reference Rodríguez Gómez2025a; Rodríguez Gómez et al. Reference Rodríguez Gómez, Vieira, Dal Lago and Palacios2018; Rodríguez-Gómez 2017). It uses the relationship between the magnetic field, density, temperature, and emission. This model adequately describes emissions coming from the solar corona on scales ranging from days to solar cycles.
SSI time series on long time scales are important for feeding ITM models, e.g., Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model (TIME-GCM; Roble and Ridley Reference Roble and Ridley1994), the NCAR Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM-X; Liu et al. Reference Liu, Bardeen, Foster, Lauritzen, Liu, Lu, Marsh, Maute, McInerney, Pedatella, Qian, Richmond, Roble, Solomon, Vitt and Wang2018), and the Whole Atmosphere Model-Ionosphere Plasmasphere Electrodynamics Model (WAM-IPE; Akmaev Reference Akmaev2011), among others. These models can help to understand the role of SSI in the upper Earth’s atmosphere.
Taking advantage of the excellent performance of the CODET model, providing SSI in EUV wavelengths covering the last solar cycles. This work presents the most recent results and challenges, e.g., new results using observational SSI (SDO/EVE), applications like Earth’s exospheric time series, and SSI at Mars’ distance. As well as future work, e.g. modelling SSI from other stars to characterize exoplanet atmosphere, and SSI from young Sun stages.
2. The COronal DEnsity and Temperature (CODET) model
The CODET model was developed by Rodríguez-Gómez (2017). This model uses photospheric magnetic flux from SOHO/MDI (Scherrer et al. Reference Scherrer, Bogart, Bush, Hoeksema, Kosovichev, Schou, Rosenberg, Springer, Tarbell, Title, Wolfson, Zayer and Team1995), and SDO/HMI (Scherrer et al. Reference Scherrer, Schou, Bush, Kosovichev, Bogart, Hoeksema, Liu, Duvall, Zhao, Title, Schrijver, Tarbell and Tomczyk2012), a flux transport model from the Lockheed Martin Solar and Astrophysics Laboratory (LMSAL) (Schrijver Reference Schrijver2001) and the Potential Field Source Surface (PFSS) (Schrijver and De Rosa Reference Schrijver and De Rosa2003; Schrijver Reference Schrijver2001) coronal magnetic field extrapolation is used to retrieve the solar atmosphere’s magnetic structure. The plasma temperature and density are obtained using scaling laws that depend on the magnetic flux. Additionally, density and temperature are used as inputs to the emission model based on the CHIANTI atomic database Footnote † to retrieve the SSI in EUV wavelengths. Its first version; the CODET model version 1.0 (Rodríguez Gómez et al. Reference Rodríguez Gómez, Vieira, Dal Lago and Palacios2018) uses TIMED/SEE data to compare model outputs. A recent version, namely the CODET model version 1.1 (Rodríguez Gómez Reference Rodríguez Gómez2025a), uses SDO/EVE data to constrain SSI from the model and thus provides time series close to observational data. The CODET model supplies various products, including daily SSI time series, density and temperature maps and profiles, and full disc emission maps. Figure 1 shows the updated time series from the CODET model v 1.1 from July 1, 1996, to September 3, 2025 (Rodríguez Gómez Reference Rodríguez Gómez2025a). These time series provide SSI predictions after the observational gap in 2014, when the SDO/EVE MEGS-A detector malfunctioned, and before 2010. The outliers (peaks) appearing in the time series predictions of Rodríguez Gómez (Reference Rodríguez Gómez2025a) are related to some artifacts in the flux transport models. They were replaced by the mean value from the day before and the day after the outlier occurrence (Figure 1).
Solar Spectral Irradiance (SSI) prediction using COronal DEnsity and Temperature model v1.1 from July 01, 1996, to September 3, 2025. Top: Solar Spectral Irradiance from CODET version 1.1 at
$28.4 \ \mathrm{nm}$
(gray line), EVE/SDO (red line), and EUVS/GOES (blue line). Bottom: SSI from CODET version 1.1 at
$21.1 \ \mathrm{nm}$
(gray line), EVE/SDO (magenta line), and AIA daily mean values (green line).
Figure 1 Long description
Two line graphs depict solar spectral irradiance over time for different wavelengths and instruments. Panel A: The top graph shows irradiance in W/m^2/nm on the vertical axis and time from 2000 to 2025 on the horizontal axis. It includes data from CODET 28.4 nm (gray line), EVE/SDO (red line), and EUVS/GOES (blue line). The graph shows fluctuations in irradiance with notable peaks around 2000, 2014, and 2020. Panel B: The bottom graph shows irradiance in W/m^2/nm on the vertical axis and time from 2000 to 2025 on the horizontal axis. It includes data from CODET 21.1 nm (gray line), EVE/SDO (magenta line), and AIA daily mean values (green line). This graph also shows fluctuations in irradiance with peaks around 2000, 2014, and 2020.
3. Applications for the study of planetary atmospheres
SSI affects Earth’s atmosphere, e.g., the exosphere. This layer is important because it can provide clues about the past and future of the Earth’s atmosphere. Especially because Earth loses part of its atmosphere every day due to the Sun-Earth interaction. The exosphere connects Earth’s atmosphere to interplanetary space. Thus, Earth’s exosphere holds important information on this loss mechanism (Rodríguez Gómez Reference Rodríguez Gómez2025b; Connor et al. Reference Connor, Jung, Claudepierre, Mierkiewicz, Zhang, Pham, Cucho-Padin, Dayeh, Bhattacharyya, Nossal and Sutton2023). Solar irradiance plays a crucial role in determining exospheric density, but there are no direct measurements of SSI at exospheric heights over long time scales. Thus, modeled SSI becomes an important tool for studying Earth’s exosphere. Rodríguez Gómez (Reference Rodríguez Gómez2025b) provided a new SSI time series from the CODET
${\mathrm{R_e}}$
model. These time series were obtained at different geocentric distances (
$d_{\mathrm{R_e}}$
), e.g.,,
$3.0 \ \mathrm{R_e}$
,
$6.6 \ \mathrm{R_e}$
and
$8.0 \ \mathrm{R_e}$
where Re corresponds to the Earth’s radius. Those heights are important for evaluating exospheric models, e.g., Cucho-Padin et al. (Reference Cucho-Padin, Bhattacharyya, Sibeck, Connor, Youngblood and Ardila2023) and Connor et al. (Reference Connor, Jung, Claudepierre, Mierkiewicz, Zhang, Pham, Cucho-Padin, Dayeh, Bhattacharyya, Nossal and Sutton2023).
The influence of SSI is present even in the early planetary stages, some planets could lose most of their water during the early active period of the young Sun e.g., Mars and Venus (Rodríguez Gómez Reference Rodríguez Gómez2025b and references therein). The current structure and description of the CODET model can be used to obtain SSI at Mars distance. It is an adapted version of the CODET model versions
$1.0$
and
$1.1$
at
$28.4 \ \mathrm{nm}$
and
$21.1 \mathrm{nm}$
. These time series were obtained by integrating them at an average distance of
$\sim 1.5 \ \mathrm{AU}$
between the Sun and Mars, and compared to MAVEN/EUVM data (Rodríguez Gómez Reference Rodríguez Gómez2025b). SSI at Mars distance can help to understand solar variability and its impact on its atmosphere. This is important for future exploration of Mars and exoplanets.
4. Discussion
The COronal DEnsity and Temperature (CODET) model adaptability and capabilities allow for obtaining SSI time series at Earth’s exosphere and at different heliocentric distances, e.g., at
$1.0 \ \mathrm{AU}$
and
$\sim 1.5 \ \mathrm{AU}$
(Mars distance). These time series provide an exceptional opportunity to study planetary atmospheres, even when no observational data (SSI) are available. This model is still in constant development and improvement, and some of its key aspects can be summarized as follows.
-
• It is important to highlight that the CODET model provides a unique SSI from coronal lines with temperature formation
$ \geq \log_{10} \mathrm{T} \sim 6 \ \mathrm{K}$
. Thus, it is not possible to obtain the solar chromospheric or photospheric emission with the current physical description (scaling laws) of this model. New scaling laws related to the photospheric magnetic field and chromospheric emission will be explored in the future, as well as a Machine Learning (ML) approach to obtain SSI from chromospheric lines, e.g., He II
$30.4 \ \mathrm{nm}$
, C III
$97.7 \ \mathrm{nm}$
, and O VI
$103.2 \ \mathrm{nm}$
, etc. -
• SSI from the CODET model can be improved in future versions by incorporating the solar magnetic field contributions from the far side. Also, including the planetary or satellite position, e.g., heliocentric distance, SPICE/SoLO, STEREO/EUVI, and the planetary position like Mars using the Stonyhurst Heliographic (HGS) system, longitude, latitude, and the radial distance coordinate from the Sun’s center for a specific planet or spacecraft.
-
• This model description can be used to model irradiance from other stars when the magnetic field maps and their light curves in EUV wavelengths are available. It can be a valuable tool for studying stellar variability and exoplanet atmospheres. Using the current model setup, it is possible to obtain stellar irradiance, e.g., using a unique stellar spot over the disk by utilizing solar photospheric magnetograms with a specific threshold that allows for the acquisition of a similar stellar magnetic field, e.g., younger suns
$\sim 120 \ \mathrm{Myr}$
with average magnetic field strength from
$1.3$
to
$2.0 \ \mathrm{kG}$
and
$0.2 - 0.8 \ \mathrm{kG}$
in older stars (Kochukhov et al. Reference Kochukhov, Hackman, Lehtinen and Wehrhahn2020). It can allow for exploring and modeling SSI during the young sun period, providing clues about stellar/sun evolution and the early stages of planetary structure and dynamics.
28.4 nm
21.1 nm
Open access
